Most prior art ink compositions for use in ink jet printers employed either a dye or a printer which incorporated a mechanical device for keeping pigments dispersed. The dyes, depending on the color, often faded quickly, and the mechanical devices often proved to be less than the ideal, i.e. to develop a pigmented composition which can be used in the ink jet printing systems currently commercially available. The several pigmented compositions which I have found are disclosed in the following patents.
U.S. Pat. No. 5,310,778, of May 10, 1994, assigned on its face to duPont, discloses that the preparation of ink jet inks is improved by using a dispersion of pigment and polymeric dispersant in an aqueous carrier medium, and then employing a milling step. The dispersant is a block copolymer having a molecular weight below 10,000 prepared from acrylates or methacrylates or mixtures thereof. It is said that the particle size ranges from 0.005-1.mu.. The carrier medium disclosed is water and at least one water-soluble organic solvent, present in the amount of 65-99.5%, based on the total weight of the ink. The pigment is present in an amount up to 30%, and the polymeric dispersant in the amount of 0.1-30%, based on the total weight of the ink composition. The viscosity ranges from 1 to 10 cp. and the surface tension ranges from 30 to 70 dynes/cm, both at 20.degree. C.
Another duPont patent, U.S. Pat. No. 5,418,277, dated May 23, 1995, discloses an ink particularly adapted for ink jet printers. It contains a fluorinated block copolymer prepared from fluorinated oxazoline or oxazine in an amount of 0.005 to 10%. The aim is to get good drying times and print quality. The ink contains an aqueous carrier medium, a colorant and the copolymer, which is a hydrophobic block containing at least one fluorine atom and a hydrophilic block.
duPont U.S. Pat. No. 5,519,085 of May 21, 1996, discloses a triblock coplymer as dispersant, where the triblock consists of a hydrophilic polymer block (A), a polymer block particularly capable of binding solids (B), and a third polymer block which can be hydrophobic or hydrophilic (C). A and C are end blocks.
U.S. Pat. No. 5,537,137 of Jul. 16, 1996 discloses a reactive media ink system where the media has a coating which contains a hydrophilic component and a reactive compound which, upon exposure to an external energy source, becomes more durable.
U.S. Pat. No. 5,281,261 of Jan. 25, 1994 employs a polymerized vinyl aromatic salt, which is grafted to the surface of the pigment particles in an aqueous liquid vehicle comprising water and humectant. The ink has a viscosity of 10 cp. or less.
U.S. Pat. No. 5,514,207 of May 7, 1996 is mainly concerned with low molecular weight (200-800) polyethylene glycol as latency extenders in pigmented ink jet inks. Sulfalone is included, in the range of about 5-40%.
U.S. Pat. No. 5,106,417 of Apr. 21, 1992 discloses an aqueous printing composition for drop-on-demand type in jet printing, where the ink contains 0.5-5% by weight solid pigment preparation (30-70% pigment and 70-30% polyacrylic resin), 2-10% water-dilutable organic solvent (C1-C4 alkanols, lower aliphatic ketones, cellosolves, carbitols, N-methyl-2-pyrrolidone, 2-pyrrolidone, N,N-dimethyl formaldehyde, N,N dimethylacetamide and mixtures thereof), 2-15% humectant (glycol, glycerin, or polyethylene glycol and water. Viscosity ranges of about 2-5 mPas.
U.S. Pat. No. 5,512,089 of Apr. 30, 1996 describes a black ink, where pigment is dispersed in an aqueous medium using a polymeric surfactant. The redispersibility of the ink is achieved by raising the pH to 9-12. The pigment has an average particle size of less than 5.mu.. The liquid vehicle is selected from the group consisting of a wetting agent, a biocide, deionized water, glycol, a water miscible cosolvent and mixtures thereof. Furthermore, a base of carbon dispersion, polymeric surfactant and water is added to a conventional humectant such as polypropylene glycol.
U.S. Pat. No. 5,316,575 of May 31, 1994 discloses an ink composition suitable for ink jet printing, comprising an aqueous resin solution and an organic pigment, where the composition is substantially free of volatile organic compounds. The ink has a basic pH, a viscosity of 1.6-7 cp. at 25.degree. C., an electrical resistivity from 50-2000 ohm/cm and a sonic velocity from 1200-1300 meters/seconds. The resin is soluble in water at a basic pH and is present in 20-60 wt %.
In none of the above patents is the relationship between surface tension and print quality sufficiently described. Neither is the requirement to achieve durability described. My invention is based on the discovery that it is essential to control surface tension in order to achieve compatibility between inks of different color. Furthermore, the compositions of the present invention achieve durability of at least 6 months outdoor exposure. This promises to broaden the range of applications in the field of ink jet printing.
Some of my earlier work is disclosed in U.S. Pat. No. 5,798,781 dated Aug. 25, 1998, of which the present application is a continuation-in-part. That patent, which is concerned with an improved printing apparatus, also contains a general description of a pigmented ink jet ink composition. It discloses a pigment dispersion, glycol, water, amine, and additives such as a surfactant, a biocide, humectant and anti-foaming agent. The ink disclosed consists of 50-95% water by weight, pigment 1-10 wt-%, cosolvent in the range of 5-35%-wt., and other additives from about 0.5 to 10 wt.-%. It is also disclosed that the surface tension should be controlled.